1. Field of the Invention
This invention relates to apparatus for subjecting materials to microwaves, and more particularly, to a novel applicator for insertion into a material in order to radiate microwave energy into the material.
2. Description of Related Art
Various applicators for use in heating materials with microwaves are known. For example, C.P. No. 1,044,331 issued Dec. 12, 1978 to Hamid discloses a microwave horn applicator which may be placed on frozen soil in order to thaw the soil. It is disclosed that, in thawing soil, the depth of microwave penetration is limited to about 2.5 inches for soil at -10.degree. F. and to about 5 inches for soil at 20.degree. F.
U.S. Pat. No. 4,339,648, issued July 13, 1982 to Jean discloses another applicator, a hollow slotted rectangular waveguide which may be inserted into a confined material in order that microwave energy may be radiated into the material. This has the advantage that the depth of penetration depends on the length of the applicator and not the depth of microwave penetration as with an applicator placed on the surface of the material. Further, the area of penetration is governed by the number and the spacing of the applicators. However, a disadvantage with a hollow waveguide of any configuration is that it must meet certain minimum dimensional requirements in order to propagate microwave energy. More particularly, one cross-sectional dimension of any hollow waveguide must be at least equal to half the wavelength of the microwave energy applied to the waveguide in order for microwave energy to propagate along the guide.
Specific microwave frequency bands have been allocated for industrial, scientific and medical use of microwaves. Of the allocated bands, 915 MHz.+-.13 MHz and 2,450 MHz.+-.50 MHz are common in the use of microwaves for heating purposes. Relatively inexpensive microwave generators exist for these frequencies.
At 915 MHz, microwaves have a one-half wavelength of about 6.3 inches and at 2,450 MHz a one-half wavelength of about 2.4 inches. If frozen ground is to be thawed by means of an insertable applicator, a hole must be provided in the frozen soil to accommodate the applicator. Such a hole may be drilled. For a viable industrial operation, the diameter or the required hole should not be greater than about 2 inches. Consequently, a hollow waveguide would not be suitable for this purpose.
The minimum dimensions of a hollow waveguide may be reduced by filling the guide with dielectric material. This, however, is an unacceptable solution to the minimum dimensional requirements because of the resultant power losses in the dielectric material.
In result, there is a need for an insertable microwave applicator for heating materials operating in at least the 915 MHz.+-.13 MHz and 2,450 MHz.+-.50 MHz bands and with an outside cross-sectional diameter not exceeding 2 inches. The invention herein provides such an applicator.